Absence of spin-density excitations in quasi two-dimensional electron systems

Inelastic light scattering spectra reveal a new behavior of electron-electron interactions in two-dimensional double quantum well semiconductor structures. It manifests in intersubband excitations when both subbands are densely populated. In such high density limit, spin excitations, well defined when only one subband is populated, cannot be distinguished from single-particle transitions. A quantitative analysis shows a collapse of vertex corrections (exciton shift), which we attribute to the cancellation between contributions from electrons in the two subbands.